The lock box could also house a regular 12V deep cycle battery and simple controller. The top could serve as the platform for a small solar panel. The solar panel keeps the battery charged up, and the battery supplements/charges the outboard battery, both while it's running and when it's not running (using a 12V battery supplement is described in the Torqeedo literature).

Depending on usage of the engine, this might be a self contained system needing little or no outside charging.